1. Field of the Invention
The present invention relates to layouts used to fabricate a phase shifting mask (PSM) and a trim mask, and in particular to various modifications that can be made to the PSM layout and the trim mask layout to provide critical dimension (CD) control, even in non-critical areas.
2. Description of the Related Art
An advance in lithography called phase shifting is able to generate features on the wafer that are smaller than the corresponding wavelength of the light. These ultra-small features are generated by the interference of light in adjacent, complementary pairs of phase shifters having opposite phase, e.g. 0 and 180 degrees. In one embodiment, the phase shifters can be formed on a phase shifting mask (PSM), which is used in conjunction with a trim mask to define features of a layout. In the PSM, complementary phase shifters (hereinafter referred to as shifters) are configured such that the exposure radiation transmitted by one shifter is 180 degrees out of phase with the exposure radiation transmitted by the other shifter. Therefore, rather than constructively interfering and merging into a single image, the projected images destructively interfere where their edges overlap, thereby creating a clear and very small image between the phase shifters.
The PSM has been used to define certain critical edges of a feature, whereas a light field trim mask has been used to define the other edges of the feature. For example, the PSM can be used to define the gates of an integrated circuit, whereas the trim mask can be used to define the other edges in the integrated circuit. More recently, the PSM has been used to define substantially all features on the layout including both critical and non-critical features. For example, in one type of PSM called a full phase PSM (FPSM), a trim mask can protect areas defined by the FPSM as well as,expose undesirable features that may print because of phase assignments on the FPSM.
For example, FIG. 1A illustrates an exemplary layout 100 for a FPSM, wherein this layout defines a T-intersection. Layout 100, which is implemented in a dark field mask, includes two 0 degree shifters 101 and 104 as well as two 180 degree shifters 102 and 103. Shifters 101 and 103 define one line of the T-intersection, shifters 102 and 104 define another line of the T-intersection, and shifters 101 and 102 define yet another line of the T-intersection.
Note that the phase assignments discussed herein are illustrative only. The important aspect is that shifters on opposite edges of a line have a phase difference of approximately 180 degrees. Thus, shifters 101 and 104 could be 180 degree shifters, whereas shifters 102 and 103 could be 0 degree shifters. Moreover, shifters 101 and 104 could be 185 degree shifters, and shifters 102 and 103 could be 5 degree shifters. To conform to this configuration in a T-intersection, one corner can include one shifter (e.g. shifter 101) and another corner can include two shifters (e.g. shifters 103 and 104).
A cut 105 between shifters 103 and 104 solves a potential phase conflict when assigning phase to the T-intersection (i.e. a “cut” separates two shifters that in the absence of a phase conflict could be implemented as a single shifter), but results in an extraneous feature when exposed. A trim mask layout 110, shown in FIG. 1B, can erase this extraneous feature (i.e. by exposing the photoresist in that area). Specifically, trim mask layout 110 includes a cut 111 to account for the proximity of shifters 103 and 104 in the right corner of FPSM layout 100. Of importance, cut 111 on trim mask layout 110 actually defines the right corner of the feature (i.e. the T-intersection). The other areas of trim mask layout 110 protect the features defined by FPSM layout 100.
FIG. 1C illustrates an aerial image 120 (determined by simulation) assuming a double exposure using masks implementing FPSM layout 100 and trim layout 110. In image 120, the following parameters were used: a wavelength of 193 nm, a numerical aperture (NA) of 0.7, and a partial coherence factor (σ) of 0.5. In this case, the trim mask is exposed to twice the energy of the FPSM (i.e. a 1:2 exposure ratio). In other words, if the FPSM is exposed to N mJ/cm2, then the trim mask is exposed to 2N mJ/cm2.
The blue portion of image 120 indicates a low intensity, the red portion indicates a high intensity, the yellow portion indicates an intermediate intensity, etc. The high intensity correlates to a high exposure, whereas the low intensity correlates to a low exposure. As evidenced by the thin band of yellow in image 120, the transition from high to low intensity is abrupt, thereby resulting in well-defined features. However, the described corner cut in the trim mask can cause the right and left corners of the T-intersection to print differently (i.e. cause CD variations).
FIG. 2A illustrates a graph 200 that plots the CD error versus the distance to a polysilicon line for various misalignments of the FPSM mask and the trim mask. (For reference, FIG. 2B illustrates the desired T-intersection that is to be produced using the masks created from the layouts of FIGS. 1A and 1B, wherein a CD is measured at a cut line 201 taken at a distance 202 from a polysilicon line 203.) The indicated measurements in FIG. 2A, i.e. the CD errors, distances, and misalignments are in nanometers. The misalignments shown in the legend (to the immediate right of FIG. 2A), d(0,0), d(10,10), etc., refer to (x,y) misalignments of the PSM and trim mask relative to each other. Graph 200 shows that as the distance increases (i.e. moving away from the corner of the T-intersection), the CD error decreases irrespective of mask misalignments. However, the mask misalignments can cause significant CD errors, particularly near the corners.
FIG. 2C illustrates a graph 210 that plots the CD versus the distance to the poly line (both measurements in nanometers). In this example, the CD decreases (e.g. the layout is changed and the corresponding PSM and trim mask are revised as well) from approximately 135 nm to approximately 115 nm as the distance to the polysilicon line increases from 75 nm to 200 nm. In other words, the CD at a corner is significantly greater than the nominal CD, e.g. in this case, 100 nm, but approaches the nominal CD as the effects of the corner rounding decrease.
Therefore, a need arises for a technique in a full-phase PSM that provides CD control at line intersections while at the same time minimizes sensitivity to mask misalignment.